Right breast hurting, did a self and didn't feel anything beside maybe by my arm pit. mastitis, breast cancer, or something else?

cancer unlikely: Breast pain is poorly understood. Breast cancer rarely presents as breast pain. See your doctor. Mastitis would be much more common for a woman in her mid 20's.

Cystic change: Fibrocystic disease/changes is very common in women. This is most often seen during the menstrual cycle. Breast feel lumpy and painful which is caused by cyst formation (fluid filled areas) due to hormonal changes. Some women may have these changes without being on their menstrual cycle. Caffeine may cause the cysts to be overactive resulting in pain. This is not related to cancer.

Probably not cancer: Pain under your arm and in your upper breast is probably not a breast cancer. Pain in the breast is very common. If it comes and goes with your periods, i'm reassured. If it is constant or worsening, you should see a doctor. There is a rare type of breast cancer called inflammatory breast cancer that causes severe pain, redness, swelling, and skin dimpling, but your symptoms sound more mild.